This invention relates in general to ascertaining electrical discharge properties of electrophotographic imaging members and more specifically, to apparatus and process for measuring the potential across a photoconductive layer during cycling using an electrostatic meter.
In the art of electrophotography an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is image by first uniformly electrostatically charging the imaging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such a light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated area. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
The flexible photoreceptor belts are usually multilayered and comprise a substrate, a conductive layer, an optional hole blocking layer, an optional adhesive layer, a charge generating layer, and a charge transport layer and, in some embodiments, and anti-curl backing layer.
Although excellent toner images may be obtained with multilayered belt photoreceptors, it has been found that as more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, there is a need to better characterize the photoreceptors. The photoreceptor characteristics that have a bearing on the ultimate print quality include: charge acceptance when contacted with a given charge, the dark decay in the rested (first cycle) and fatigued state (steady state after a few cycle), the discharge of Photo Induced Discharge Characteristics (PIDC) which is the relationship between the potential remaining as a function of light intensity, the spectral response characteristics, and the residual potential in addition, during cyclic operation in apparatus such as a copier, duplicator of printer, a photoreceptor may undergo conditions known as cycle-up or cycle-down. Cycle-up is a phenomenon in which residual potential and or background potential keeps increasing as a function of cycles. This generally leads to increased and unacceptable background density in copies of the documents. Cycle-down is a phenomenon in which the dark development potential (potential corresponding to unexposed regions of the photoreceptor) keeps decreasing as a result of increased dark decay as a function of cycles. This generally leads to reduced image densities in the copies of the documents. Thus, there is a need to measure all these photoreceptor characteristics with ease and over a wide range of timings, temperatures and ambient conditions.
Cycling scanners employing corotrons have been utilized for measuring photoreceptor characteristics. These scanners are designed to simulate the cycling of photoreceptors in a copier, duplicator and printer by subjecting a test sample of photoreceptor to timed charge, expose and discharge cycles. Scanners do not utilize all of the stations in a completely operational xerographic machine. Thus, for example, test scanners normally involve electrical charging, imagewise discharging and flood erase steps omitting the development, transfer and cleaning steps. In drum scanners the photoreceptor in the form of a cylindrical drum (or belt pieces mounted on a drum blank) is rotated on a shaft. The photoreceptor is charged by means of a corotron mounted along the circumference of the drum. The surface potential is measured as a function of time by several capacitively coupled probes placed at different locations around the drum. The probes are calibrated by applying a known potential to the drum substrate. The photoreceptor is exposed and erased by light sources located at appropriate positions around the drum. The measurement involve charging the photoreceptor in a constant current (a certain charge is placed on the photoreceptor) or a constant voltage mode. As the drum rotates the initial charging potential is measured by a first probe. Further rotation lead to an exposure station where the photoreceptor is exposed to a monochromatic or broad band light of known intensity. The surface potential after exposure is measured by a second and third probe. The photoreceptor is finally exposed to an erase lamp of appropriate intensity and any residual potential is measured by a fourth probe. The process is repeated with the magnitude of the exposure automatically changed for the next cycle. A photo induced discharge curve is obtained by plotting the potential at the second and third probes as a function of exposure. Further experimentation might involve changing the wavelength of the exposure and repeating the procedure or eliminating the exposure and measuring the dark decay. Cyclic stability of the photoreceptor can be measured by continuous cycling for 10,000 to 100,000 cycles.
Components of the drum scanner are mounted so that corotron, exposure lamp and probes can be moved along the circumference of the drum and clamped. The shortcomings of this type of system include, for example, the time under corotron (or the voltage source) is limited to the physical width of the corotron divided by the surface velocity of the drum which might range between 5 inches per sec to 60 inches per second. Also the voltage is measured at four (or any other number equal to the number of probes employed) discrete points in time determined by the angular location of the probe (with respect to the corotron) divided by the surface velocity. Further, it is cumbersome to move probes to change timing. In addition, data relating to potential between the charging station and the first probe is not available. Moreover, the physical size of drum scanners requires that the scanner equipment be placed in a large controlled atmosphere chamber which in turn requires a long time to change ambient (relative humidity and temperature) conditions. Further, drum scanners cannot be operated in non air (e.g. nitrogen or argon) environments to study the role of oxygen in photoreceptor operation or degradation. Also, corona charging is unstable in nitrogen or argon atmospheres. In drum scanners, the maximum potential is limited by what the charging device will allow.